Field of the Invention and Related Art Statement
The present invention relates to a circuit for correcting the tone of an image signal, and more particularly to a circuit for correcting automatically the tone of a brightness signal of a television signal generated by a television camera.
When picking-up scenes with the aid of the television camera, the image signal is supplied to a television monitor to display images on a monitor screen. Nowadays the brightness conversion characteristic of the television monitor has changed. That is to say, almost all television previously monitors had gamma values of about 2.2, but recently the gamma value has been increased to about 2.8. When the image signal is supplied to such a television monitor having the higher gamma value, an image displayed on the display screen of the monitor is deteriorated such that the tonal property of a relatively dark portion of the displayed image becomes worse. In order to avoid such a deterioration of the tone of the displayed image, there has been proposed a technique for compensating for the deterioration of the tone by correcting the tone of the image signal such that the gain of the brightness signal is selectively increased only at dark areas. Such a tone correcting circuit has been disclosed in Japanese Patent Application Laid-open Publication Kokai Sho 60-214,692. In the known tone correcting circuit described in this publication, the brightness signal generated by the television camera is supplied to a non-linear circuit, i.e. a so-called knee circuit and the gain of the brightness signal in the dark area is enhanced to derive an enhanced brightness signal, a tone correction signal is produced by deriving the difference between the original brightness signal and the enhanced brightness signal, and the tone correction signal thus derived is added to the original brightness signal.
In the above explained known tone correction circuit, the whole brightness signal is processed by the non-linear signal, so that the gain of the brightness signal in the bright area is decreased by the non-linear circuit and the contrast of the brightness signal in the bright area is also decreased. Further, when the correction amount of the brightness signal in the dark area is increased, the black level is also increased and thus the contrast of the displayed image is correspondingly decreased. Therefore, when a scene which does not include a dark area is picked-up by the television camera, the whole displayed image becomes white and the image quality is deteriorated to a large extent.
In order to remove the above mentioned drawbacks of the known tone correcting circuit, consideration has been given to manually adjusting a gain control amplifier for amplifying the tone correction signal. Then the operator watches carefully the displayed image to determine whether tone correction is required or not, and when the quality of the image displayed on the monitor screen has been made worse by the tone correction, the gain of the correction signal amplifier is adjusted to zero. Such a manual adjustment is apparently very cumbersome for the operator. Moreover when the operator is in such a situation that he cannot use a television monitor, it is not possible to adjust the gain of the amplifier. When the operator controls the gain of the tone correction signal amplifier without monitoring the displayed image, the quality of the image might deterioration still further.
Furthermore, in the known tone correcting circuit, the tone correction signal always has a positive polarity and is added to the original brightness signal to increase the level of the image signal in the dark area. However, it has been confirmed that in some case it is desired to decrease the level of the brightness signal in the dark area in order to perform a special production effect. For instance it is sometimes desired to darken the dark background to increase the brightness of a subject in a scene. The known tone correcting circuit can not perform such a special production effect.